22 General utilities library [utilities]

22.5 Optional objects [optional]

22.5.4 Partial specialization of optional for reference types [optional.optional.ref]

22.5.4.2 Constructors [optional.ref.ctor]

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template<class Arg> constexpr explicit optional(in_place_t, Arg&& arg);
1
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Constraints:
  • (1.1)
    is_constructible_v<T&, Arg> is true, and
  • (1.2)
    reference_constructs_from_temporary_v<T&, Arg> is false.
2
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Effects: Equivalent to: convert-ref-init-val(std​::​forward<Arg>(arg)).
3
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Postconditions: *this contains a value.
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template<class U> constexpr explicit(!is_convertible_v<U, T&>) optional(U&& u) noexcept(is_nothrow_constructible_v<T&, U>);
4
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Constraints:
  • (4.1)
    is_same_v<remove_cvref_t<U>, optional> is false,
  • (4.2)
    is_same_v<remove_cvref_t<U>, in_place_t> is false, and
  • (4.3)
    is_constructible_v<T&, U> is true.
5
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Effects: Equivalent to: convert-ref-init-val(std​::​forward<U>(u)).
6
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Postconditions: *this contains a value.
7
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Remarks: This constructor is defined as deleted if reference_constructs_from_temporary_v<T&, U> is true.
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template<class U> constexpr explicit(!is_convertible_v<U&, T&>) optional(optional<U>& rhs) noexcept(is_nothrow_constructible_v<T&, U&>);
8
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Constraints:
  • (8.1)
    is_same_v<remove_cv_t<T>, optional<U>> is false,
  • (8.2)
    is_same_v<T&, U> is false, and
  • (8.3)
    is_constructible_v<T&, U&> is true.
9
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Effects: Equivalent to: if (rhs.has_value()) convert-ref-init-val(*rhs);
10
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Remarks: This constructor is defined as deleted if reference_constructs_from_temporary_v<T&, U&> is true.
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template<class U> constexpr explicit(!is_convertible_v<const U&, T&>) optional(const optional<U>& rhs) noexcept(is_nothrow_constructible_v<T&, const U&>);
11
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Constraints:
  • (11.1)
    is_same_v<remove_cv_t<T>, optional<U>> is false,
  • (11.2)
    is_same_v<T&, U> is false, and
  • (11.3)
    is_constructible_v<T&, const U&> is true.
12
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Effects: Equivalent to: if (rhs.has_value()) convert-ref-init-val(*rhs);
13
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Remarks: This constructor is defined as deleted if reference_constructs_from_temporary_v<T&, const U&> is true.
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template<class U> constexpr explicit(!is_convertible_v<U, T&>) optional(optional<U>&& rhs) noexcept(is_nothrow_constructible_v<T&, U>);
14
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Constraints:
  • (14.1)
    is_same_v<remove_cv_t<T>, optional<U>> is false,
  • (14.2)
    is_same_v<T&, U> is false, and
  • (14.3)
    is_constructible_v<T&, U> is true.
15
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Effects: Equivalent to: if (rhs.has_value()) convert-ref-init-val(*std::move(rhs));
16
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Remarks: This constructor is defined as deleted if reference_constructs_from_temporary_v<T&, U> is true.
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template<class U> constexpr explicit(!is_convertible_v<const U, T&>) optional(const optional<U>&& rhs) noexcept(is_nothrow_constructible_v<T&, const U>);
17
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Constraints:
  • (17.1)
    is_same_v<remove_cv_t<T>, optional<U>> is false, and
  • (17.2)
    is_same_v<T&, U> is false.
  • (17.3)
    is_constructible_v<T&, const U> is true,
18
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Effects: Equivalent to: if (rhs.has_value()) convert-ref-init-val(*std::move(rhs));
19
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Remarks: This constructor is defined as deleted if reference_constructs_from_temporary_v<T&, const U> is true.